1. Field of the Invention
The present invention relates to an electrochemical cell having a conductive, dimensionally stable, oxide growth resistant current distributor. The current distributor of the present invention is useful in a process for converting anhydrous hydrogen halide to a halogen gas or in an aqueous electrochemical process. The oxide growth resistant current distributor of the present invention is particularly useful in the very aggressive environment associated with the oxidation of HCl to Cl.sub.2, whether in an anhydrous or an aqueous process.
2. Description of the Related Art
Hydrogen chloride (HCl) or hydrochloric acid is a reaction by-product of many manufacturing processes which use chlorine. For example, chlorine is used to manufacture polyvinyl chloride, isocyanates, and chlorinated hydrocarbons/fluorinated hydrocarbons, with hydrogen chloride as a by-product of these processes. Because supply so exceeds demand, hydrogen chloride or the acid produced often cannot be sold or used, even after careful purification. Shipment over long distances is not economically feasible. Discharge of the acid or chloride ions into waste water streams is environmentally unsound. Recovery and feedback of the chlorine to the manufacturing process is the most desirable route for handling the HCl by-product.
A number of commercial processes have been developed to convert HCl into usable chlorine gas. See, e.g., F. R. Minz, "HCl-Electrolysis--Technology for Recycling Chlorine", Bayer AG, Conference on Electrochemical Processing, Innovation & Progress, Glasgow, Scotland, UK, Apr. 21-23, 1993.
Currently, thermal catalytic oxidation processes exist for converting anhydrous HCl and aqueous HCl into chlorine. Commercial processes, known as the "Shell-Chlor", the "Kel-Chlor" and the MT-Chlor" processes, are based on the Deacon reaction. The original Deacon reaction as developed in the 1870's made use of a fluidized bed containing a copper chloride salt which acted as the catalyst. The Deacon reaction is generally expressed as follows: ##STR1## where the following catalysts may be used, depending on the reaction or process in which equation (1) is used.
______________________________________ Catalyst Reaction or Process ______________________________________ Cu Deacon Cu, Rare Earth, Alkali Shell-Chlor NO.sub.2, NOHSO.sub.4 Kel-Chlor Cr.sub.m O.sub.n MT-Chlor ______________________________________
The commercial improvements to the Deacon reaction have used other catalysts in addition to or in place of the copper used in the Deacon reaction, such as rare earth compounds, various forms of nitrogen oxide, and chromium oxide, in order to improve the rate of conversion, to reduce the energy input and to reduce the corrosive effects on the processing equipment produced by harsh chemical reaction conditions. However, in general, these thermal catalytic oxidation processes are complicated because they require separating the different reaction components in order to achieve product purity. They also involve the production of highly corrosive intermediates, which necessitates expensive construction materials for the reaction systems. Moreover, these thermal catalytic oxidation processes are operated at elevated temperatures of 250 C. and above.
Electrochemical processes exist for converting aqueous HCl to chlorine gas by passage of direct electrical current through the solution. The current electrochemical commercial process is known as the Uhde process. In the Uhde process, aqueous HCl solution of approximately 22% is fed at 65.degree. to 80.degree. C. to both compartments of an electrochemical cell, where exposure to a direct current in the cell results in an electrochemical reaction and a decrease in HCl concentration to 17% with the production of chlorine gas and hydrogen gas. A polymeric separator divides the two compartments. The process requires recycling of dilute (17%) HCl solution produced during the electrolysis step and regenerating an HCl solution of 22% for feed to the electrochemical cell. The overall reaction of the Uhde process is expressed by the equation: ##STR2## As is apparent from equation (2), the chlorine gas produced by the Uhde process is wet, usually containing about 1% to 2% water. This wet chlorine gas must then be further processed to produce a dry, usable gas. If the concentration of HCl in the water becomes too low, it is possible for oxygen to be generated from the water present in the Uhde process. This possible side reaction of the Uhde process due to the presence of water, is expressed by the equation: EQU 2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+ +4e.sup.- ( 3)
Further, the presence of water in the Uhde system limits the current densities at which the cells can perform to less than 500 amps./ft..sup.2, because of this side reaction. The side reaction results in reduced electrical efficiency and corrosion of the cell components.
Another electrochemical process for processing aqueous HCl has been described in U.S. Pat. No. 4,311,568 to Balko. Balko employs an electrolytic cell having a solid polymer electrolyte membrane. Hydrogen chloride, in the form of hydrogen ions and chloride ions in aqueous solution, is introduced into an electrolytic cell. The solid polymer electrolyte membrane is bonded to the anode to permit transport from the anode surface into the membrane. In Balko, controlling and minimizing the oxygen evolution side reaction is an important consideration. Evolution of oxygen decreases cell efficiency and leads to rapid corrosion of components of the cell. The design and configuration of the anode pore size and electrode thickness employed by Balko maximizes transport of the chloride ions. This results in effective chlorine evolution while minimizing the evolution of oxygen, since oxygen evolution tends to increase under conditions of chloride ion depletion near the anode surface. In Balko, although oxygen evolution may be minimized, it is not eliminated. As can be seen from FIGS. 3 to 5 of Balko, as the overall current density is increased, the rate of oxygen evolution increases, as evidenced by the increase in the concentration of oxygen found in the chlorine produced. Balko can run at higher current densities, but is limited by the deleterious effects of oxygen evolution. If the Balko cell were to be run at high current densities, the anode would be destroyed.
U.S. Pat. No. 4,294,671, also to Balko, discloses another configuration for an electrochemical cell for processing aqueous HCl. In this cell, niobium current distributing screen elements are positioned between an anode deposited on a membrane and an anode current collector. Metals such as niobium, tantalum, titanium, etc. and alloys thereof are known to have good corrosion resistance and conductivity. However, they are costly. Moreover, valve metals also have a tendency to passivate by the formation of protective surface oxide layers which are very poor conductors. Hence, it is necessary to coat the valve metal with a non-oxide forming material such as a film of one of the platinum group metals, which further adds to the cost. In addition, the platinum group metals corrode in the presence of chlorides and oxidizing potentials, as is the case when generating chlorine for HCl. As a result, known current collectors use costly materials, require difficult and costly manufacturing procedures and in many ways present problems from a fabrication and cost standpoint.
It is also known to use graphite for a current collector, as disclosed in U.S. Pat. No. 4,294,671 to Balko, or a graphite-polyvinylidene fluoride, sold under the trademark KYNAR.RTM. by Elf Atochem North America, Inc. Fluoropolymers, as disclosed in U.S. Pat. No. 4,214,969 to Lawrance. However, it has been found that graphite can be oxidized, due to the side reaction of oxygen generated from water, as expressed in equation (3) above. Moreover, graphite and the best case of graphite-polyvinylidene fluoride have resistivities of 10.sup.-3 ohm.multidot.cm and 3.times.10.sup.-3 ohm.multidot.cm, respectively, which make them relatively poor conductors.
Oda et al., in U.S. Pat. Nos. 4,909,912, 4,666,574 and 4,655,887, recognize the corrosion-resistant properties of a porous layer made of the oxides, hydroxides, nitrides or carbides of certain metals in electrolyzing an aqueous solution of an alkali metal chloride. The porous layer is disposed between a membrane and an anode, which in turn is disposed in contact with the alkali metal chloride. Thus, although corrosion resistant, the porous layer does not act as a corrosion barrier to the alkali metal chloride. Nor does it act as an electronic current conductor, instead acting as an ionic current transmitter.